One problem facing China in all this is "what about the border?". For now, or at least until recently, China had a convenient 'buffer state' between themselves and the perfidious West/South Koreans. OK, NoK is a loose cannon, but unlikely to start firing things at China.
If NoK is destabilised or dismantled, what will take it's place? Will SoK occupy [peacefully or otherwise] NoK, thus bringing them [and with the US in company] right up to the Chinese border?
So from China's perspective, which is the lesser evil?